The etching of silicon wafers by "plasma" or "dry" techniques is still more of an art than a science. A well defined etching process trys to meet four main criteria simultaneously: (1) high etch rate(s) of the material or materials to be patterned, (2) low etch rates of the mask and etch stop materials, (3) good anisotropy and (4) good etch rate uniformity. Often it is not possible to achieve all four at the same time. In situations for which the selectivity and anisotropy are merely adequate, it is desirable to etch only until the pattern is defined. Over-etching leads to unacceptable line width control. Consequently one wants to stop the etching as close to the point of completion as possible.
One of the most useful methods for monitoring the etching of a wafer is to monitor the amplitude intensity of optical emission from the plasma discharge. Many processes have particular spectral lines or regions which are driven by the presence of some chemical constituent in the plasma such as a reactant species or a species produced as an etching by-product. By tracking the intensity at the wavelength of these species in the plasma, one has some idea about the concentration of these species in the plasma which is directly related to the status of the etching process.
One known apparatus monitors a single spectral line at a time. However, it monitors that line with two channels. The first channel monitors the light intensity in a narrow region about the predetermined spectral line while the second channel monitors the light intensity in a substantially larger bandwidth which is close to, but does not include, the spectral line. Accordingly, the first channel monitors the intensity of the line of interest while the second channel monitors the background signal. By subtracting the background signal from the first channel signal the intensity of the desired line may be obtained.
If the emisson from an etching plasma has a line in the region of the spectrum with low background or, more accurately, if the line strength is very high with respect to the background, then it is a simple matter to monitor the process. However, when the spectral emission does not have a line strength that is substantially higher than the background, problems arise in distinguishing the line from the background resulting in underetching or overetching of the devices being processed.